Grass trimmers have been around for over 30 years, starting with gas (high speed 2-cycle engine) power and later evolved to both AC electrics and very limited cutting capable DC (battery) cordless trimmers. Practically every household, industrial, or institutional property with grass or weeds relies on the versatility of the trimmer to finish trim its continually growing vegetation. The working effort with trimmers is performed by ‘do it yourself’ property owners or by professional landscapers. Approximately 10-12 million trimmers are sold annually within the United States alone.
The cutting elements used with trimmers are for the most part an extruded flexible nylon based monofilament line or string (of only one cross section—e.g., round, square star, etc.) that is oriented (stretched) which ‘orients’ its molecular chain to its maximum extension to achieve its maximum tensile strength. As is it stretched, its cross-section is diminished. Other less commonly used cutting attachments are both brittle plastic or combined with metal flail-like blades, but non-flexible blades that pivot on a pin. Brittle plastic blades shatter when hitting into solid obstacles. The flexible cutting line (or fishing string) frequently and continuously breaks and/or wears quite easily requiring continuous manual replenishment or from pre-wound spools integrated into a centrifugal, or bump feed systems, themselves quite complex. Due to complex advance and feed systems, fixed line systems have been popular with specially designed but costly systems needing constant manual replenishment of the rapidly wearing or breaking conventional line.
As a cutting tool, the line trimmer has been very popular, but has its own inherent problems that have to date remained unsolved. These problems have created great frustration to the user, and further have created limitations to the design of the trimmer itself, and the resulting lack of ergonomics, excess power consumption, reliability, cost, noise, vibration, and more.
The thin, low mass and weak structure of the monofilament line (cutting element) must rotate at very high tip speeds (e.g., 30,000 ft/min) in order to achieve its cutting stiffness with the speed dependant centrifugal force. As a result, the operating stresses exceed the material properties causing continuous failure and need for constant replenishment. The high speed line at impact to solid objects (rocks, fences, concrete) creates the excess stress and continuously breaks. Further, in addition to its poor cutting and edging performance, the symmetrical cross section requires high power input to push itself through the high drag of the air and grass drag causing poor trimming inefficiencies and excess power consumption. Consequently, the power driving system needs to be continuously powerful and high speed capable.
Monofilament line with such low mass and weak structure will easily fold back when spinning due to its limited strength, which further diminishes its cutting capability and quality of cut (involving grass to cause browning) Adding more line mass add to more aerodynamic drag losses. Additionally, grass drag increases the more it folds back. Accordingly, the low mass cutting line must be spinning at very high tip speeds to become a better cutting mechanism from the added centrifugal forces created by the necessary higher speeds. This process takes even more energy to drive the line through the air due to added air drag which increases by multiple powers of the higher velocities. Further, due to constant impact of the line with solid or hard objects, line breakage is certain and rapid thus requiring constant replenishment. Overall, this cutting structure creates inefficiency in power consumption, wasted power due to air and grass drag, limited or poor cutting quality and capability, and constant line replenishing harassments for the user. To solve these long and historic problems will be greatly desirable.
Often at the outer tip of the swath where the cutting occurs, the line frays and splits into ‘fibrels’ back along the nylon co-polymer's molecular chain. Additionally, nylon is moisture dependent and when it dries due to lower humidity environments, it has a short shelf life, as it becomes brittle to then rapidly break.
Noise (from the high speed engine and a high pitch line squeal) is also an irritating experience to both the operator and the neighborhood, often resulting in an unpleasant and irritating trimming experience, in addition to, the imposition of local and state ordinances that are restrictive to noisy engine use.
Electric trimmers are limited by the required lengthy and interfering extension cord to reach areas of the property needing trimming. Additionally, they are also subject to many of the same weak line frustrations. Homeowners (of smaller sized lots) who dislike engines (fuels, starting smoke, noise, etc) prefer the AC electric trimmer.
Due to these many inherent monofilament design problems, cordless or battery trimmers have never been able to operate in a practical sense up to the necessary speed levels required for cutting vegetation. Battery trimmers have to run generally slower tip speeds due to the limited energy carrying capacity of their batteries. Consequentially, the high required tip speed for the cutting line causes very high drag losses thus stealing and wasting the desirable energy needed to cut vegetation. Therefore, not only running time is sacrificed but also the cutting line cannot deliver adequate severing force, and further, is unable to edge with appreciable depth. Adding more mono-filament line outlets adds to the drag, further causing battery drain. Additionally, to replenish worn or broken line normally involves either tap or centrifugal advance for feeding line out from a storage spool. This frustrating process also requires another substantial energy waste to advance and replenish the weak line on a continuous basis.
Battery or cordless trimmers have only a single cutter outlet of very thin line, and become further diminished with larger cutting swath diameters. To increase the width and mass of the line, and to increase the number of cutter outlets will elevate the cordless trimmer to much higher esteem and working capability. Additionally, the limited energy storage capacities of batteries further limit running time, require lengthy and frequent charging times, and spare batteries are very costly.
Cordless trimmer motors have been predominately, if not all, “ground mounted” at the “dirty end” of the trimmer to eliminate drive cable frictional losses. Problems arise in that the motor's air cooling system clogs and overheats with the added exposure or dirt and debris. Further, the added suspended weight of the motor creates operator fatigue and discomfort. There are great advantages in relocating the motor toward the top end of the trimmer.
As the line speed drops, the cutting capability or severing forces are diminished. One analogy would be like cutting with a “wet noodle” as opposed to a “blade-like” cutter needed to slash through the vegetation intended to be cut. Further, cutting with a slower single line diminishes the frequency of cutter contact, thus extending the inefficiencies in trimming. Consequently, conventional line from a battery trimmer cannot deliver the grass contact frequencies of the forces needed to cut the variety of vegetation needed to be cut. The force delivery is nearly “benign” and the duration of its delivery is severely limited from the many conflicting factors such as battery size, weight, cost, air and grass drag, suspended weight, clogging, only to mention a few.
To increase the cutting line force at the vegetation cutting zone, and to improve battery life by better motor operational efficiency, U.S. Pat. No. 6,014,812 utilizes step-down gearing directly adjacent to its lower or ground mounted motor. This allows a DC motor to operate at its higher speed peak efficiency while gearing the tip speed down to about 24,000 ft/min., still well below the 30,000 ft/min. needed for monofilament line to cut better. However, the ground-mounted motor system still must operate in the ‘dirty zone’ and its suspended weight must be supported by the operator with fatigue and poor ergonomics.
Today, cordless outdoor tools such as chain saws, grass trimmers, blowers, and hedge clippers are manufactured and sold as complete units, i.e., motor, battery, handles, are connected directly to the specific cutting element. While the expensive batteries are interchangeable between the individual tools, the tools are essentially individual units. This practice is due to the inherent nature of poor cutting due to limited torque delivery systems needed to sever vegetation, and the available power limitations from battery systems.
Hand held trimmers are suspended from the operator's arms and have to be maneuvered to the area to be cut. The awkward handles, combined with the need to pull the trigger often present an uncomfortable, fatiguing, and unnatural gripping effort. The hands have to maneuver the trimmer into an angle or attitude to reach the vegetation to be trimmed or edged. Combining these strenuous movements along with the trimmer's suspension weight and irritating vibration, add to further discomfort and rapid fatigue.
Some trimmers are provided with head tilt and turning provisions to help reduce the operator difficulty. However, these adjustments require the operator to make the adjustments while away from the normal operating zone.